1. Field of the Invention
This invention relates to an improved method of synthesizing a porous crystalline material designated Zeolite ZSM-22.
2. Description of Related Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure, as determined by X-ray diffraction pattern, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as having a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed by the relationship of aluminum to the cations, wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K, Cs or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. The aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-22 (U.S. patent applications Ser. Nos. 373,451 and 373,452, both filed on Apr. 30, 1982), zeolite ZSM-23 (U.S. Pat. No. 4,076,842), ZSM-35 (U.S. Pat. No. 4,016,245), and ZSM-38 (U.S. Pat. No. 4,046,859).
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often varible. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 mole ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one example of such zeolite wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5, up to infinity. U.S. Pat. No. 3,941,871, now Re. 29,948, the entire contents of which are incorporated herein by reference, discloses a porous crystalline silicate zeolite made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5 type zeolites. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294, the entire contents of all three patents being incorporated herein by reference, describe crystalline silcates or organosilicates of varying alumina and metal content.
Zeolite ZSM-22 has heretofore been synthesized with an organic compound (also known in the art as an organic promoter) expressed by the formula: EQU R.sub.4 J.sup.+
wherein J is an element of Group VB of the Periodic Chart of the Elements, e.g., N or P, preferably N, and each R is an alkyl or aryl group having at least two carbon atoms or hydrogen. Suitable organic compounds were dialkylammonium compounds wherein each of the alkyl groups was the same or different, with each alkyl group having two (2) to eight (8) carbon atoms (see, e.g., the aforementioned U.S. patent application Ser. No. 373,451, filed on Apr. 30, 1982). When alkane diamines were used as the organic compounds, it was preferred to use larger alkali metal cations, e.g., potassium (K.sup.+) or cesium (Cs.sup.+), at the SiO.sub.2 /Al.sub.2 O.sub.3 mole ratios of about 20 to about 90 to obtain ZSM-22 crystals substantially free of impurities or other crystals (see e.g., the aforementioned U.S. patent application Ser. No. 373,452, filed on Apr. 30, 1982). The use of the potassium cation may be disadvantageous in some applications because potassium is more difficult to remove than sodium from the synthesized zeolite by ammonium ion (NH.sub.4 .sup.+) exchange. As is known in the art, the presence of alkali or alkaline earth cations in zeolites lowers catalytic activity thereof.